Heating unit



AIL

Jan. 15, 1957 M. NATHANSON 2,777,930

HEATING UNIT Filed April 28, 1954 F761]. F1612. g2 32 7 I W/lo I g 22?)I J 5 5a 56 I Q 58 54 6 INVENTOR Max llfizifianson ATTORNEYS UnitedSates This invention relates to electrical heating devices. Moreparticularly, this invention relates to electrical heating deviceshaving resistance circuits imprinted upon a non-conducting base, and tomethods of producing such fixtures.

It has previously been proposed to print electrical circuits on anon-conducting base and such a technique has had wide usage in certainapplications in the field of electronics but, as yet, such circuits haveproved themselves unacceptable for simple resistance heating when theamount of resistance, and hence the Wattage output of the device iscritical. In domestic, as well as in some laboratory applications, thewattage output rating must be uniform and fall within carefully definedlimits. Industries specify high standards of control in their heatingequipment, and the underwriter laboratories demand that a similarstandard be applied to such devices sold for domestic usage. Evenwithout the high standards set by consumers, or those controlling themarket, it is nevertheless desirable to provide resistance heatershaving a carefully controlled Wattage output, because if the resistanceis too low the initial wattage will frequently burn out the circuit andrender the apparatus inoperative.

One of the major difficulties encountered with printed circuitresistance heaters is the problem of control of the cross-section of theresistance path. The metallic ink may either run or skip when printed,and the thickness frequently varies substantially through a givencircuit. Thus, a given section may be too wide or too narrow, too thickor too thin. While a portion of these inaccuracies produce compensatingerrors, the end result is that the circuit of a given design section andlength may have a resistance considerably at variance with that of thedesign specifications.

It has also been determined that circuits printed of metallic materialupon a non-conducting base have a high resistance-temperaturecoefficient. As a cold metallic circuit has a resistance considerablylower than the same circuit when heated, either by its own operation orby externally applied heat, such a heating unit constructed to maintaina wattage output below a specific quantum will, if it does not exceedthe specific standard when cold, have a considerably lower output whenit is at its normal operating temperature.

For example, a heater with certain aluminum elements producing 1000watts under a 118 volt potential will draw approximately 8.6 ampereswhen operating at its normal hot temperature. However, if such anelement is energized when cold, it will draw 11 amperes and will have anoutput of 1298 watts. As it is the practice of the underwriterslaboratories to rate the heating elements on the basis of initial inputwattage, it will be readily seen that during a major portion of theoperation of the element, the full rated output is not available.

It is therefore an object of this invention to provide a method ofconstructing printed heating circuits having a wattage output conformingto precisely defined standards.

It is another object of this invention to provide a reatent O ar C6sistance heating unit having circuits printed on a nonconducting base.

It is a further object of this invention to provide a printed circuitelectric heating unit having a controlled output so that a given Wattageis maintained both when the unit is cold and when the unit is at itsoperating temperature.

It is still a further object of this invention to provide a heatingdevice adapted for installation flush with a wall board panel.

These and other objects of the invention will be fully understood fromthe following detailed description of a typical preferred form andapplication of the invention, and the appended claims.

The invention may be best understood with reference to the accompanyingdrawings, in which:

Figure 1 is a plan view of a printed circuit illustrating the method ofmaking a precision correlation of the total resistance;

Figure 2 is a plan view of a heating element provided with a thermalcontrolled shunt adapted to maintain a uniform wattage;

Figure 3 is a plan view of a heating panel;

Figure 4 is a section taken along lines 4-4 of Figure 3; and

Figure 5 is an elevational view, taken in section, of a heating paneldisposed within a section of wall board.

In Figure 1, there is shown a printed resistance circuit which isparticularly adapted for use as an electrical resistance heating unit.The base 10 may be a sheet of ordinary paper, plastic, or glass paper,such as is manufactured of glass fibers, or any other non-conductingmaterial suitable for receiving printing or painting, spraying orscreening, with a metallic, or other conductive paint or ink. Acontinuous circuit 12, which may be of any size or shape, but is hereillustrated a square spiral, is printed or painted or sprayed orscreened upon base 10 with an ink or paint which, when dried, leaves aconductive residue or path. The ink or paint may consist of fluidshaving metallic material suspended therein, or may be composed of othermetallic compounds, including metallic salts. While any metallic paintor ink may be used, those containing silver, copper, or aluminum havebeen found to be particularly suitable. The ink or paint may be brushed,screened or printed on the base in any convenient manner.

The circuit of the heater illustrated consists of dual paths, disposedin a narrowly spaced parallel relationship, with each pair of pathsformed to a diminishing square spiral, at the end of which the paths arejoined at 11. The printed spiral circuit terminates at contact junctures14 and 16, which contain standard fittings which may be readilyconnected with usual wiring or conduit arrangements found in domesticand commercial establishments.

The choice of the material selected as a base sheet will be determinedby the temperature at which the resistance or heater is expected tooperate. For example, for temperatures up to F. many grades of ordinarypaper will be entirely satisfactory. However, for a heater operatingabove this figure, a glass paper, manufactured of glass fiber, will bemore acceptable. If the temperature is expected to reach 300 to 400 R,either glass paper or glass plate will be preferable. For highertemperatures, glass, ceramic, asbestos or similar high temperatureinsulators may be used.

As discussed hereinbefore, it is very diflicult to provide a printedresistance circuit having an even and accurately depositedcross-sectional area, and hence the wattage output obtained from a groupof similar heaters will vary considerably. In accordance with thisinvention, the printed circuit is made with an overall circuit lengthapproximately to greater than required by the design specifications.That is to say, experience 'willshow that a circuit having a particularresistance will usually have a given approximatelengths The circuit isprinted having a le'ngth'at least 5%- to 10% greater than'the figurethus determined. The normal operating potential is then impressed uponthe terminals 1 and 16 and the unit is allowed to heat upto itsoperatingtemperature. A wattmeter .1811 is incorporated into the circuit and,with the 10%, more or less, extra resistance, the indicated Wattage willbe above the desired rating of the heating unit. Test prodsjoinedby alow resistance lead are then pressed into contact with the circuit onadjacent portions of the parallel spirali circuit, for example at 13audit). Asa portion of the resistance of the printed circuit-Will beshunted out or short circuited,--the wattage output of a heater will bereduced a commensurate amount, and thischange will beindicated on thewattmeter 18a which is interposed in the feed circuit. If, however, thereduction in the wattage does not bring the rating .to the desired orspecified output of the heater, the :testprods may be moved 1 furtheralong the circuit to shut oif or short circuit increasing portionsthereof.' If, for example, when the test prods reachthe points 22, 24,the wattrneter indicates the desired or specified output of the heater,the proper location for a permanent shunt, as will be discussedhereinafter, may be considered to be determined. The voltage impressedupon terminals 14 and i6 is removed, and a short thick low resistanceshunt 2a is printed or painted between these points. Excellent resultshave been obtained by painting one or more layers of a thick metallicsolution with an ordinary paint brush, although it is within thecontemplation-of this invention to provide any low resistance shunt,such-as a metal bar. It is desirable that the heater be again brought toits operating temperature and that thewattage'output be given a finalcheck, although the use of this method allows a very small chance oferror or mistake.

As discussed above, one of the disadvantages of the use of printedcircuits inthe creation of electrical resistance heating elements hasbeen that theconductive paints or inks used, such as copper, aluminum,-or silver, all possess a high resistance-temperature coefiicient. As theelectrical heating units are rated'by underwriters laboratories. interms of initial wattage'output when cold, and as such heaters. arefrequently used for protracted periods, it will i be readily seen-thatthe full or rated output is achieved during a very small fraction ofthe-total time in which a particular heating unit is in use. Thus itwill be readily seen that the heating potential, or 'the totalcapacityto produce thedesircd quantums -of heat, is being .Wasted during,a major portion of-a given normal operating. period.

it is-p-withirr the-contemplation of this invention to providemeanswhich will-eliminate this useless waste ofthe full heatingpotential of the elements;- This end -is achieved byproviding thecircuit with-a thermal operated relay'interposed in a shunt which shor-tcircuits a portion of the resistance path of theelement, When, forexample, a-printed aluminum heating element rated by the underwriterslaboratories at 1300 watts isinitially charged with an electricalpotential, the wattage output will be at a level substantially equal tothe full rated figure. However, as the element warms up, the resistanceof the circuit rises'due to the highresistance-temperature coefiicientofthealuminum, and as the resistance rises the total wattage outputwill} the voltage remaining constant, diminish. After a very shortoperating period, the total wattage output of the heating unit will bereduced to the neighborhood of llififl'w'attswhich is only 77% of thefull rated capacity which might be expected from the apparatus and ofwhich the apparatus is capable of producing."

irtFigure 2; there' is illustrated an exemplary electrical heatingelement haviu'ga resistance circuit printed .witha conducting paint orink" upon a non-conducting" base 32'.

disposed in'a narrowly spaced parallel relationship, with each pair ofpaths formed to a diminishing square spiral at the end of which thepaths are joined, at The circuit is provided with a correlationcorrection shunt 36 painted across the dual parallel spaced paths asdiscussed hereinabovc. The dual paths are also joined by a shunt 38having a thermal-operated relay or thermostatic switch interposedtherein. Such a relay 4. 0 may consist of a bimetal element formed oftwo plates of metal having dissimilar coeificients of heating expansion.Such thermaloperated relays are in themselves old and the details of itsconstruction constitute no part of this invention. The relay tl,disposed on the shunt 38, will close the circuit of the shunt when therelay proper is heated to a predetermined level, and when it closes theremaining portion of the heating circuit, generally indicated at 42, isshort circuited and rendered inoperative. The shunting out of portion 42of the printed circuit results in an overall reduction in the-totalresistance of the heating unit, arid thus, under a given fixedpotential, the wattage output of the entire unit will be raised. inoperation, the heating unit disclosedin Figure 2 will, when cold,provide a continuous circuit from terminals 28 around the spiral to thepermanent shunt 36 The resistance to the flow of the current in themetallic paths will rapidly raise the temperature of the heating elementto a point Where the thermal-operated relay will close the circuit ofthe shunt 33,--sothat then the flow of the current will be restricted tothe parallel paths between the terminals 28 and the relay operated shunt38.' The location of thisrelay may be selected by the trialtesting withthe low resistance test prod circuit and the wattmeter as discussedhereinabove.

In Figure 3, there is shown a heating panel formed area plurality, forexample, four, heating sections 56- which-may be'the equivalent of theheating units described hereinabove in the discussion of Figures 1 and2.-

That is, the heating sections are formed of dual paths disposed innarrowly spaced parallel relationship with each set of paths formed tocreate a diminishing square spiral. Each of the heating sections 56 hasone of these dual paths connected to a first bus line, and the other ofits dual-paths connected to a second bus line. Adjacent-each heatingsection 56, there is disposed a broken printed circuit 5S-extendingbetween the bus lines 54. The break-or void 6!) in this circuit may beformed with tabs or'wide strips 55 which may be used as bases forterminal posts, as described hereinafter. It will be seen that-eachof'the heating sections 56 is spaced a substantial distance from-itsneighboring sections, so that the entire heating panel may be bisectedor otherwise divided to provides traction of the total wattage output onthe entire panel where circumstances dictate the provision of aheatingpanel having a reduced outputwattage. For example, if each of-thefour heating sections 56 have a rated wattage output of 250 watts, thecomplete panel will serve as a lOOO-watt heater. However, in certainapplications, itm'aybe desirable to provide a domestic orcommercial"establishment with a panel heater having a rated Wattageoutput-of 750 watts. Therefore, in accor-dance with this-invention, thepanel maybe cutor sawed' along sline 62,- to remove heating section 56',leaving-three heating sections having a total'wattage output of750watts.

Regard-lessof whether a panel carries one or more heating sections 56,only one set of contact junctures or terminal posts need be provided aseach of the tabs 55 is connected by a printed path or circuit with a busline 54 which, in turn, is connected with each operative heatingsection.

Referring more particularly to Figure 4, there is shown an exemplaryillustration of the composite panel in which the base sheet of glasspaper 50 has the heating elements 52, comprising the various heatingsections 56 and bus lines 54, printed thereon. A second or cover sheet64 is disposed on the printed side of the base sheet 50, and a sheet ofheat-reflecting material, such as aluminum foil 66, is disposed on theback of the base sheet. As discussed hereinabove, the base sheet and thecover sheet may be composed of paper, plastic, or plate glass, or anyother non-conducting material, and the heating element may be composedof any metallic or non-metallic conducting ink or paint.

Still referring to Figure 4, the assembled panel, that is the superposedglass paper cover, printed base sheet, and heat-reflecting sheet, aremolded or otherwise bonded into a flat unitary structure. This may beaccomplished by dipping the composite structure in a varnish, such as asilicone varnish, or by impregnating the composite structure with asuitable plastic material and applying the necessary quantum of heatand/ or pressure.

When the composite structure is so impregnated and molded, it is withinthe contemplation of this invention to form any of a variety of shapes.For example, hot cups or heated trays may be easily formed by the properselection of a suitable mold.

In Figure 5, there is illustrated an exemplary utilization of theprinted resistance heating panel. An ordinary panel of wall board ofinsulating materail, for example, GYPROC, may be provided with niche 72formed therein which is cut with a depth substantially equal to thethickness of the heating panel illustrated in Figures 3 and 4. Thisheating panel is disposed Within the niche, with the aluminum foil heatreflecting barrier 66 next to the floor or bottom of the niche. As theniche has a depth substantially equal to the thickness of the heatingpanel, the exposed side of the wall board panel 70 will present asubstantially smooth surface which, when painted or papered, willpresent an appearance virtually indistinguishable from the otherordinary wall board insulating panels. The heating panel may be gluedinto the niche, or may be secured therein by nails driven through theborder or outer edge portions 55 of the heating panel, or it may besecured therein by brackets or any other suitable detent means.

It is within the contemplation of this invention to provide panelshaving heating sections 56, which are either of the ordinary printedresistance type or of the preferred examples illustrated in Figures 1and 2 and discussed hereinabove.

Having described only a typical preferred form and application of theinvention, it is not to be limited or restricted to specific detailsherein set forth, but I Wish to reserve to myself any variations ormodifications that may appear to those skilled in the art and fallingwithin the scope of the following claims.

I claim:

1. An electrical heating panel comprising a base sheet of non-conductingmaterial, a resistance circuit printed on a first side of saidnon-conducting material, a cover sheet of non-conducting materialdisposed on the printed side of said base sheet, an aluminum foil sheetdisposed on the second side of said base sheet, said base sheet, saidcover sheet and said foil being bonded together into a panel structure,and said printed circuit comprising a plurality of bus lines extendingsubstantially the full length of said panel, and a plurality of heatingsections disposed between said bus lines, the said heating sectionsbeing spaced apart so that the panel may be bisected by cutting it alonga line extending between adjacent heating sections to reduce the wattageoutput of the panel by removing one or more heating sections therefrom.

2. A method of making an electrical element having accurately controlledresistance comprising printing the resistance circuit upon anon-conducting base, said circuit having a greater terminal-to-terminallength than required, shunting out increasing portions of the printedcircuit until the correct resistance exists between said terminals, andestablishing a permanent low resistance shunt about said portions bydepositing a substantial strip of metallic liquid upon said base, andcuring said liquid.

References Cited in the file of this patent UNITED STATES PATENTS1,696,684 Knopp Dec. 25, 1928 2,100,327 Getchell Nov. 30, 1937 2,205,543Rideau June 25, 1940 2,503,457 Speir et al Apr. 11, 1950 2,512,875Reynolds June 27, 1950 2,518,807 Musgrave et al Aug. 15, 1950 2,598,878Ballou June 3, 1952 2,613,306 Waltersdorf et al Oct. 7, 1952 2,641,675Hannahs June 9, 1953 2,660,659 Sarno Nov. 24, 1953 2,683,673 SilversherJuly 13, 1954

